LOW VOLTAGE ILLUMINATION SYSTEM

Description

FIELD OF THE INVENTION

[0001] This invention relates to power supplies for low voltage illumination.

BACKGROUND OF THE INVENTION

[0002] Owing to the prevalence of halogen lamps, low voltage illumination is becoming increasing popular and offers the advantage of small bright lamps together with increased safety in the event of contact with the supply terminals. In particular, the use of low voltage lends itself to track lighting and cable lighting systems, using halogen lamps or other low voltage lamps, which can be moved along a fixed track mounted on the ceiling so as to be directed to those places where illumination is most required.

[0003] Various proposals for low voltage illumination are known in the art. Most employ a transformer for transforming the high electrical supply voltage (typically 110 V or 220 V) to a low voltage in the order of 12 V. Since the size of the transformer is dependent on its power rating, such transformers are necessarily bulky. It is therefore known in the field of a.c. illumination to invert the incoming electrical supply voltage using a conventional rectifier and chopper circuit so as to obtain a pulsating a.c. voltage source having a high frequency in the order of 30 KHz. The use of high frequency permits the size of the transformer to be greatly reduced.

[0004] In one known arrangement a central transformer provides power at low voltage (typically 12V) to a track, rail or cable system to which low voltage lamps can then be directly connected. However, it has been found that such a proposal, although attractive, is subject to the drawback that, at high frequency, the tracks operate like a transmission line, radiating energy. This problem is exacerbated as the length of the track is increased, when the inter-conductor gap is increased and when the current flow is increased. Such drawbacks would clearly be overcome by employing a low d.c. voltage source for feeding the tracks, but this is subject to its own problems, in particular relating to the efficiency of rectification.

[0005] The conventional manner to produce d.c. voltage from an incoming a.c. voltage source is to rectify the a.c. voltage using a passive rectifier employing conventional bipolar rectifier diodes. Such diodes are typically made from silicon having a forward bias voltage of 0.7 V. Thus, the power dissipated by each rectifier diode during the half cycle that it conducts is equal to the product of the forward bias voltage of 0.7 multiplied by the current passed by the diode. In a typical arrangement for energizing a series of low voltage halogen lamps having a combined current consumption of 25 A, the power loss across each rectifier diode would thus amount to 17.50 W which, when combined with the losses in the chopper circuit and step-down transformer, is unacceptable.

[0006] In this regard it is to be noted that inefficient rectification of the output not only increases operating costs but also results in heat dissipation requiring that the physical size of the power supply be increased. This, of course, militates against the very reason for using high frequency in the first place: namely to reduce the size of the power supply. Therefore the conventional approach to rectification, which is widely applied in other systems, is not suitable for halogen lighting due to the use of low voltages and hence high currents and the necessity of keeping power losses down so as to enable a small physical size.

[0007] In one popular arrangement, the low-voltage power is applied to two uninsulated conductors in the form of cables or rigid rails to which the lamps are attached. Such an arrangement is subject to the hazard that an electrically conductive short, particularly a thin wire, which is accidentally applied between the two conductors may conduct potentially large currents. The resultant heating of the wire constitutes a fire hazard. This situation manifests itself as an increase in current which may either be detected using a fuse which burns out when the heating effect of the current flow exceeds an allowed threshold, by a heat-sensitive electronic device, or by a more sophisticated current sensing element which is more directly responsive the current flow. For example, U.S. Patent No. 5,523,653 discloses a low voltage lighting fixture connected to an isolation step-down transformer. The fixture is protected from limited or maximum short circuit conditions by monitoring the secondary current of the step-down transformer until a fault is detected, whereupon the protection circuit de-energizes the primary of the transformer.

[0008] DE 37 35 989 published May 3, 1989 in the name of Berne-Electronic GmbH discloses a switching power supply for a low-voltage halogen lamp, with a radio interference suppression circuit, having a rectifier which supplies voltage to a DC voltage input, with two push-pull semiconductor switches coupled by means of transformer coils and constituting a high frequency generator, and a voltage-dependent switching element, coupled to the DC voltage input, for the triggering of a semiconductor switch.

[0009] The technical problem to which this document relates is to configure the switching power supply so as to guarantee a maximum degree of radio interference suppression. The radio interference suppression circuit contains two rod-core chokes, and the parameters of the two semiconductor switches are as nearly identical as possible.

[0010] DE 41 12 676 published December 19, 1991 in the name of Asea Brown Boveri AG describes a power supply for low voltage levels having an energy-loading primary side, an energy-emitting secondary side and a switched-in high frequency transformer. Two SRSC switches in push-pull configuration are provided on the secondary side. Each SRSC switch includes a saturable core element connected in series with a synchronous rectifier switch comprising a pair of power MOSFETs, which are switched on and off synchronously with the secondary voltage of the transformer. The saturable core constitutes an upper limit for a short-circuit current. Behind the SRSC switches in push-pull configuration are a freewheeling diode and an LC filter. The at least two SRSC switches operate in push-pull and are both in the ON state at the same time for a given time interval.

[0011] In such a circuit, a separate saturable core element is required for each MOSFET in the synchronous rectifier. The essential feature of this circuit lies in allowing a short-circuit to take place, but setting an upper limit for the short-circuit current by means of the saturable core element. The inductance of the secondary coil of a high frequency transformer can thus be made as low as possible, because it no longer has to be used as a current limiter. The lower inductance, however, also shortens the commutating time and thereby reduces the losses related to commutation.

[0012] By "commutating time" is meant the dead taken to trigger an initially non-conducting MOSFET after an initially conducting MOSFET is switched off. It is explained that the gate unit operates so that when it transpires, as a result of the detected voltage signals, that, for example, the first SRSC switch is in the OFF state and the voltage to that SRSC switch has a positive neutral continuity, it must be switched ON. Because of the saturable core element, this happens with a certain delay. As soon as the voltage to the saturable core element becomes negative (and a negative current flows in the saturable core element), the MOSFET of this SRSC switch is turned OFF again.

[0013] Thus the saturable core introduces a minimum time delay between the switching OFF of one MOSFET and the switching ON of the other MOSFET. During this time delay ("dead time") neither MOSFET conducts and this is undesirable.

[0014] None of these solutions is entirely satisfactory because none operates instantaneously when the power rating of the load connected to the supply exceeds the power rating of the supply itself. Specifically, even in the more sophisticated case where current itself is monitored, since the supply voltage is alternating, the current must climb from zero to the danger threshold before the protection element can operate. Even in this brief time interval during the 50/60 Hz cycle, the electrically conductive short can reach dangerous temperatures.

SUMMARY OF THE INVENTION

[0015] It is therefore an object of the invention to provide a low voltage illumination system which addresses the drawbacks associated with hitherto proposed low voltage illumination systems.

[0016] According to the invention, there is provided a low voltage illumination system, comprising:

connection terminals for connecting to a source of low frequency a.c. 20 voltage,

a frequency converter coupled to the source of a.c. voltage for converting the low frequency a.c. voltage to high frequency a.c. voltage, modulated by a full wave rectified lowfrequency envelope

a step down transformer coupled to an output of the frequency converter for converting to high frequency, low voltage a.c.,

a synchronous rectifier coupled to a secondary of the step down transformer for converting the high frequency low voltage a.c. to a low magnitude nominally d.c. voltage, and

a pair of conductors connected to said low magnitude nominally d.c. voltage for connecting low voltage lamps thereto;

   wherein
   the synchronous rectifier includes at least two MOSFETs,
   the step down transformer is coupled to an output of the frequency converter for driving said MOSFETs so as to produce rectified high frequency, low voltage pulses modulated by a full wave rectified low frequency envelope, and
   a residual storage unit is provided in respect of each MOSFET for storing charge when the respective MOSFET is OFF so as to boost a gate-to-source voltage thereof when the respective MOSFET is ON, in order that the respective MOSFET remains ON for an extended time period thereby reducing dead time at each end of the rectified a.c. half cycle between one MOSFET switching OFF and a complementary MOSFET switching ON.

[0017] Low voltage lamps may be directly connected to an output of the synchronous rectifier. Alternatively, there may be connected thereto a track comprising a pair of spaced apart low voltage d.c. conductors for connecting low voltage lamps thereto. Owing to the fact that the voltage across the track conductors is d.c., there is no limit to the spacing between the conductors which affects a.c. track lighting systems.

[0018] Preferably, the components in the illumination system according to the invention are provided in modules so that each module is compatible with complementary components made by other manufacturers. This modularity increases the versatility of the invention.

[0019] According to a preferred embodiment, the power supply includes a protection element directly responsive to the impedance of the load across the supply so as to interrupt the supply immediately the load impedance exceeded an allowed threshold. Such a protection element is designed to operate anywhere in the a.c. current cycle thus overcoming the drawbacks with conventional approaches.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to understand the invention and see how the same may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Fig. 1 is a block diagram showing the principal functional components of an low voltage d.c. illumination system according to the invention;

Fig. 2 is a schematic circuit diagram of a synchronous rectifier for use with the system shown functionally in Fig. 1;

Figs. 3a to 3d are schematic representations of various voltage waveforms associated with the synchronous rectifier shown in Fig. 2;

Fig. 4 is a pictorial representation of a planar transformer for use with the system depicted in Fig. 1; and

Fig. 5 is a schematic circuit diagram of a protection circuit for use with the system shown functionally in Fig. 1 and responsive to the load impedance across the supply for preventing overload.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0021] Fig. 1 shows a low voltage illumination system designated generally as 10 which comprises a pair of input terminals 11 and 12 (constituting connection means) for connecting to a source of low frequency a.c. voltage 13 which is shown in dotted outline since it is not itself part of the invention. The a.c. voltage source 13 is derived from a conventional electricity supply feeder having a voltage of 220/110 V and a supply frequency of 50/60 Hz. A conventional rectifier 14 is coupled via the terminals 11 and 12 to the source of a.c. voltage 13 for converting, the low frequency a.c. voltage to d.c. which is then fed to an inverter 15 containing a conventional chopper circuit for converting to high frequency a.c. at 30 KHz. The rectifier 14 in combination with the inverter 15 thus constitutes a frequency conversion means 16 for converting the low frequency a.c. voltage to high frequency a.c. voltage.

[0022] A step down transformer 17 is coupled to an output of the frequency conversion means 16 for converting the high frequency supply voltage of 220/110 V to high frequency, low voltage a.c. signal having a voltage of typically 12 V. The step down transformer 17 is preferably implemented using a toroidal ferrite core and the output winding is preferably implemented using a litz (bundle of very fine insulated wires) in order to minimize losses by reducing the leakage current due to the air gap between the primary and secondary windings and by reducing losses due to the skin-effect and proximity effect. Alternatively a higher frequency may be used and the output transformer implemented using a planar transformer as described below. Although this a.c. voltage is capable of powering low voltage halogen lamps connected to a pair of conductors coupled directly to the step down transformer 17, this is not desirable because such conductors can behave as a transmission line at high frequency resulting in significant energy loss. This is particularly manifest in the case where the conductors are configured as spaced apart rails with or without insulation, such as are used in cable lighting systems, wherein the separation of the conductors increases the transmission line effect.

[0023] To prevent the drawback associated with large high frequency currents, the high frequency signal is rectified using a synchronous rectifier 18 coupled to a secondary winding (not shown) of the step down transformer 17 for converting the low voltage a.c. to low voltage d.c. A pair of conductors 19 and 20 are connected to the low voltage d.c. for connecting low voltage lamps (not shown) thereto.

[0024] Track lighting systems per se are known and it is therefore not necessary to describe in detail all the components shown functionally in Fig. 1. Rather, with reference to Figs. 2 and 3 there will now be described those features which are specific to the present invention and offer improved performance over known systems.

[0025] Fig. 2 is a schematic circuit diagram of the synchronous rectifier 18 shown functionally in Fig. 1. The step down transformer 17 has a single primary winding 25 rated at 120 V a.c. at 30 KHz and a secondary winding 26 having a center-tap connected to a zero voltage rail 27, and on each side of which there are two taps connected to respective first and second voltage rails 28a, 29a and 28b, 29b each exhibiting a differential voltage of 12 V a.c. at 30 KHz. The circuit is symmetrical and therefore only the upper half will now be described for the positive half cycle of the a.c. voltage when the upper coil connection (shown dotted) is positive. In this case, the voltage on the first voltage rail 28a varies between zero and about +17V (corresponding to +12 V (rms)) whilst the voltage on the second voltage rail 29a varies between zero and +24 V (rms). Regardless of the instantaneous voltage on the two voltage rails 28a and 29a, there exists a differential voltage therebetween of constant polarity and rms magnitude equal to 12 V.

[0026] Connected across the first and second voltage rails 28a and 29a is a filter comprising a resistor 30 and a capacitor 31 for filtering out high frequency components. Connected to the junction of the resistor 30 and the capacitor 31 is a capacitor 32 rated at 100nF which is connected to the cathode of a rectifier diode 33 whose anode is connected to the first voltage rail 28a via a current limiting resistor 34. During the positive half cycle, the first voltage rail 28a is at +12 V (rms) and the second voltage rail is at +24 V (rms). Consequently, the rectifier diode 33 is reverse biased and non-conducting, thereby preventing discharge of the capacitor 32 through the resistor 34. However, during the negative half cycle, the first voltage rail 28a is at -12 V (rms) and the second voltage rail is at -24 V (rms). Consequently, the rectifier diode 33 is then forward biased and conducts current, thereby charging the capacitor 32. Thus, during successive negative half cycles the capacitor 32 accumulates charge.

[0027] Connected across the series connected rectifier diode 33 and resistor 34 is a resistor 35 and a pair of back-to-back connected Zener diodes 36 and 37 rated at 15 V which ensure that during both positive and negative half cycles, the differential output voltage thereacross never exceeds 15 V. The anode of the upper Zener diode 36 is connected to the gate terminal of a MOSFET 38 whose source terminal is connected to the first voltage rail 28a and whose drain terminal is connected to the 12 V d.c. output 39 of the synchronous rectifier in parallel with the drain terminal of the complementary MOSFET 40 in the lower half of the circuit.

[0028] The operation of the circuit will now be explained with reference to the voltage waveforms shown schematically in Figs. 3a to 3d of the drawings. Fig. 3a shows the voltage waveform at the input to the step down transformer 17 across the primary winding 25. It will be noted that the voltage across the primary winding 25 corresponds to a 30 KHz square wave pulse modulated by a 100 Hz full wave rectified envelope. During the positive half cycle, the voltage fed to the gate terminal of the MOSFET 38 is at approximately 24 V (rms) less some voltage dropped across the resistor 30. The voltage fed to the source terminal of the MOSFET 38 is nominally 12 V (rms) equal to the supply voltage of the first voltage rail 28a. Consequently, there exists a differential voltage of +12 V (rms) between the gate and source terminals of the MOSFET 38 which therefore conducts current, In similar manner, there exists a differential voltage of -12 V (rms) between the gate and source terminals of the MOSFET 40 in the lower half of the circuit which is therefore cutoff. During the negative half cycle, the situation reverses and the upper MOSFET 38 is cutoff whilst the lower MOSFET 40 conducts, its drain still providing the desired output of 12 V d.c.

[0029] In Fig. 3b, the full line shows the voltage waveforms on the upper first voltage rail 28a, whilst the dotted line shows the complementary situation prevailing on the lower first voltage rail 28b whose voltage is in anti-phase with the voltage on the upper first voltage rail 28a. In both cases, an instantaneous voltage sample is shown, it being understood that just as the voltage across the primary winding 25 of the step down transformer 17 is a 100 Hz modulated high frequency pulse, so too is the voltage across the respective secondary windings of the step down transformer 17 a modulated 30 KHz pulse having a fundamental frequency of 100 Hz. Thus, the absolute instantaneous voltages across the voltage rails 28a and 28b vary between zero and ± 17 V (12V rms).

[0030] Fig. 3c shows an instantaneous sample of the voltage appearing at the output 39 of the synchronous rectifier showing successive nominally square wave pulses derived alternately from the upper and lower MOSFETs 38 and 40. Fig. 3d shows the actual voltage waveform appearing across the output 39 of the synchronous rectifier comprising rectified 30 KHz pulses (i.e. at a frequency of 60 KHz) modulated by a full wave rectified 100 Hz envelope. Each pulse has a duty cycle of approximately 16 µs, there being a dead time of several microseconds at each end of the rectified a.c. half cycle owing to the time between one MOSFET switching OFF and the complementary MOSFET switching ON. If necessary, the high frequency component can be suppressed using a low pass filter 41 connected between the output 39 of the synchronous rectifier and the zero voltage rail 27. Likewise, (although not tisually necessary when only lamps are to be connected to the system) the low frequency 100 Hz ripple can be smoothed in known manner by means of a capacitor 42 also connected across the output rails.

[0031] The function of the capacitor 32 is now described. Without this capacitor there would be a considerable dead time during which both of the MOSFETs 38 and 40 would be off around the time of the zero crossing of the low frequency power source (typically occurring at a frequency of 100Hz). This derives from the fact that in order for either of the MOSFETs to conduct, there must exist a gate to source voltage V_GS of at least 8 V (assuming a pinch off voltage of approximately 4 V). This means that, under normal circumstances, a MOSFET will remain ON only whilst the differential voltage across the respective first and second voltage rails exceeds 8 V.

[0032] However, in the circuit shown in Fig. 2, the capacitor 32 allows the MOSFET to continue conducting even when the differential voltage across the respective first and second voltage rails falls below 8 V, providing that V_GS does not fall below approximately 5 V. This decrease in the dead time between switching of the MOSFETs 38 and 40 is brought about because during the negative half cycle when a MOSFET is OFF, the capacitor 32 in the corresponding half of the circuit becomes charged, as explained above, and maintains its charge because the diode 33 is reverse biased during the subsequent positive half cycle when the MOSFET is ON. Therefore, when V_GS drops below 8 V, the capacitor 32 discharges through the MOSFET which thus remains ON until the capacitor 32 fully discharges. The capacitor 32 thus constitutes a residual storage means for storing charge during the time when the MOSFET is OFF so as to boost the gate-to-source voltage during the positive half cycle when the differential voltage across the upper and lower voltage rails becomes too low to switch the MOSFET to its ON state, thereby extending the proportion of the 100Hz cycle during which the MOSFETs are able to switch ON.

[0033] It should be noted that the output of the synchronous rectifier 18 is not pure d.c. but has superimposed thereon a.c. harmonics having fundamental frequencies of twice the supply frequency and of twice the inverter frequency, respectively, as well as other weaker harmonics. Since the power supply is intended for connecting to low voltage lamps, there is no need to filter out this harmonic component. However, if desired, additional components may be provided within the synchronous rectifier for altering the waveform of the output voltage. It is thus to be understood that within the context of the invention and the appended claims, the term "low magnitude nominally d.c. voltage" is intended to imply a nominally d.c. voltage of low magnitude (i.e. compared to the supply voltage) and of low frequency compared to the very high frequency (30 KHz) associated with the step down transformer 17. In other words the object of the synchronous rectifier 18 is to render the output voltage sufficiently low frequency to avoid the transmission line radiation associated with very high frequency; whilst still allowing the use of very high frequency in the voltage conversion stage so as to reduce the bulk of the step down transformer 17.

[0034] When the MOSFET 38 conducts, its output impedance is nominally 4 mΩ, whilst when it is switched off its output impedance is in the order of several megohms. The power dissipated across the MOSFET 38, assuming an output current of 25 A, is thus equal to I²R i.e. 625 * 4 * 10^-3 = 2.5 W. As against this, if a rectifier diode having a forward bias voltage of 0.7 V or a conventional bipolar junction transistor having a base-emitter voltage V_BE equal to 0.7 V were employed at the output of the synchronous transformer 18, then the power loss would be equal to 25 * 0.7 = 17.5 W. It is thus clear that use of an active rectifier employing MOSFETs results in much higher efficiency. This increase in efficiency is particularly important when a low voltage source is employed because the lower the supply voltage, the more significant is the junction bias voltage of a conventional rectifier or bipolar junction transistor. Put another way, the lower the supply voltage, the higher is the fractional power loss across the junction, and the greater is the advantage of using a MOSFET as described.

[0035] It should be noted that in the circuit described above, the MOSFETs switch the positive output terminal. A 24V rail is therefore needed to switch the MOSFET as the gate voltage must be approximately 10V more positive than the source voltage in order to switch on the MOSFET. This requires that fixed output taps be provided on the transformer. Alternatively the MOSFETs may be connected to the negative output terminal thus requiring only three output taps of 12V (rms), 0V and 12V (rms) with negative phase on the transformer eliminating taps 26 and 27. To implement such a circuit the source and drain of each of the two MOSFETS is reversed, and the rails 29A and 28B and similarly 29B and 28A are unified.

[0036] It has been explained that a principal reason for converting from the 50/60 Hz low frequency associated with the electricity supply feeder to high frequency a.c. at 30 KHz is the reduction in size of the step down transformer which is thereby facilitated. This reduction is brought about because at high frequencies, the transformer core can be replaced by a miniaturized ferrite core assembly which can easily be accommodated in a relatively shallow ceiling recess.

[0037] Fig. 4 is a pictorial representation showing an exploded view of a planar transformer 50 which may used for effecting the step down transformation associated with the step down transformer 17 provided the inverter outputs a frequency substantially higher than 30 KHz and preferably of the order of several hundred KHz. The windings of the planar transformer 50 are constructed on a multilayer printed circuit board assembly using low thickness copper strips 51 and include leadframe windings 52 to decrease the influence of skin and proximity effects, thereby allowing higher frequencies to be employed. Full details of a planar transformer suitable for use with the low voltage illumination system 10 are provided in U.S. Patent No. 5,010,314. The use of such frequencies and of planar transformers has not hitherto been considered in lighting systems due to the fact that it is not customary to rectify the output of power supplies for lighting and, without rectification, the use of such high frequencies is unacceptable. However the advantages offered by the use of a planar transformer according to the invention, particularly the cheaper manufacturing costs, mean that such use is advantageous even if the rectifier is a passive diode bridge despite the large heat losses of such a rectifier.

[0038] Fig. 5 is a schematic circuit diagram of a protection circuit 55 for use with the system shown functionally in Fig. 1 and responsive to the load impedance across the supply for preventing overload. The protection circuit 55 includes a comparator 56 having fed to an inverting input 57 (constituting a first input of the comparator) a function of a supply voltage associated with the illumination system and having fed to a non-inverting input 58 (constituting a second input of the comparator) a function of a current flow associated with the illumination system. To this end there is connected across the supply a voltage divider comprising a pair of resistors R and r whose common junction is connected to the inverting input 57 of the comparator 56. Likewise, the output current is fed through a sensing resistor R_sense so that the voltage across the sensing resistor R_sense is a function of the supply current, I.

[0039] An output 59 of the comparator 56 goes from a logical LOW level (constituting a first state) to a logical HIGH level (constituting a second state) when a predetermined function of the impedance across the conductors falls below a predetermined threshold R'. Specifically, the output 59 of the comparator 56 goes from LOW to HIGH if:

[0040] A circuit interruption device (not shown) is responsively coupled to the output 59 of the comparator 56 for interrupting power to the pair of conductors if the impedance falls below the predetermined threshold R'. A feedback resistor 60 (constituting a latching circuit) is connected between the output 59 of the comparator 56 and the non-inverting input 58 of the comparator 56 for maintaining the output 59 of the comparator 56 HIGH regardless of a subsequent rise in impedance across the conductors. An indication lamp 61 is connected between the output 59 of the comparator 56 and GND via a current limiting resistor 62. The indication lamp 61 constitutes an indication means responsive to the impedance across the conductors falling below the predetermined threshold for indicating an active state of the protection circuit, during which power is interrupted to the conductors. Clearly, in addition to, or instead of, the lamp 61, there may be provided any other suitable audible and/or visual alarm for indicating a fault condition across the conductors.

[0041] It will be appreciated that reverse logic can be applied such that the current function is fed to the inverting input 57 and the voltage function is fed to the non-inverting input 58, in which case the interruption device is operated when the output 59 of the comparator 56 goes from HIGH to LOW.

[0042] The comparator 56 thus constitutes an impedance measuring circuit which may be incorporated within the frequency conversion means 16, preferably between the rectifier 14 and the inverter 15 or at any other location within the system.

[0043] In either case, the protection circuit 55 operates to interrupt power immediately a short circuit or near short circuit across the output of the power supply regardless of where the resulting drop in output impedance is detected in the a.c. current cycle. This is in contrast to hitherto proposed protection circuits which are directly responsive to overcurrent detection and therefore do not operate towards the start of the a.c. current cycle when the magnitude of the current waveform is still too low to constitute an overcurrent even if the peak value of the waveform is indeed dangerously high.

[0044] As is known, when lamps are operated at less than their full power rating, as when a dimmer is employed, for example, the resistance of the lamps' filaments drops owing to the lower temperature of operation. Therefore, if the illumination system is to be used in conjunction with dimming devices, then in order to ensure that the reduced impedance offered by the lamps across the conductors does not cause the protection circuit to trip the power supply, it is necessary that the predetermined threshold R' be less than the system impedance at maximum load and full dimming.

[0045] Unless steps are taken to compensate for the variation in lamp impedance owing to change in operation voltage, effectively less protection is offered at full voltage operation than at reduced voltage operation. It is therefore preferable to render the threshold R' variable based on the instantaneous value of the lamp voltage, so that if a dimming device be employed thereby reducing the lamp voltage, maximum protection will be offered for all settings of the dimming device. To this end, the value of any of the parameters R, r and R_sense may be rendered dependent on the lamp voltage appearing across the conductors. This can be done continuously by use of a suitable voltage-controlled resistor such as, for example, a FET operated in the region before pinch-off, where the drain to source voltage V_DS is small. Alternatively, discrete control can be provided by means of a bank of equal value resistors connected in parallel which are selectively switched in circuit according to the lamp voltage.

[0046] In order to provide the required selection, each of the resistors may be connected in series with a corresponding MOSFET whose gate terminals are driven by a respective threshold comparator having a threshold corresponding to a different preset voltage. By such means, the corresponding threshold comparator effects enabling or disabling of the resistors allowing the resistance of the resistor bank to be varied as required. The number of different resistance levels thus selectable is, of course, equal to 2^N where N is the number of resistors in the resistor bank.

[0047] It will be appreciated that the overload protection circuit 55 is capable of more general application to any power supply wherein immediate overload protection is required in the event of a sudden drop in output impedance owing to a short circuit or near short circuit across the output of the power supply.

[0048] In the preferred embodiment, the synchronous rectifier is a half wave rectifier using only two MOSFETs thus requiring a center-tapped transformer. Alternatively, a full bridge rectifier employing four MOSFETs may be used, thus obviating the need for a transformer with a center-tap.

[0049] It will also be appreciated that the illumination system may be contained within a common housing having lugs or other attachment means for fixing the housing to a support surface. Alternatively, the synchronous rectifier may be provided in a physically separate module to the frequency conversion means and the transformer means. Such a modular approach affords the possibility to connect a low voltage lamp to an existing high frequency, low voltage a.c. source such as a so-called electronic transformer via the synchronous rectifier module.

Claims

1. A low voltage illumination system (10), comprising:

connection terminals (11, 12) for connecting to a source (13) of low frequency a.c. voltage,

a frequency converter (16) coupled to the source of a.c. voltage for converting the low frequency a.c. voltage to high frequency a.c. voltage, modulated by a full wave rectified low frequency envelope

a step down transformer (17) coupled to an output of the frequency converter for converting to high frequency, low voltage a.c.,

a synchronous rectifier (18) coupled to a secondary of the step down transformer (17) for converting the high frequency low voltage a.c. to a low magnitude nominally d.c. voltage, and

a pair of conductors (19, 20) connected to said low magnitude nominally d.c. voltage for connecting low voltage lamps thereto;

   characterised in that:

the synchronous rectifier (18) includes at least two MOSFETs (3 8, 40),

the step down transformer (17) is coupled to an output of the frequency converter for driving said MOSFETs so as to produce rectified high frequency, low voltage pulses modulated by a full wave rectified low frequency envelope, and

a residual storage means (32) is provided in respect of each MOSFET for storing charge when the respective MOSFET is OFF so as to boost a gate-to-source voltage thereof when the respective MOSFET is ON, in order that the respective MOSFET remains ON for an extended time period thereby reducing dead time at each end of the rectified a.c. half cycle between one MOSFET switching OFF and a complementary MOSFET switching ON.

2. . The illumination system according to Claim 1, wherein:

the synchronous rectifier (18) includes a pair of MOSFETs (3 8, 40), and

the step down transformer (17) includes a center tap for driving the MOSFETs.

3. The illumination system according to Claim 1 or 2, wherein the MOSFETs have a nominal output impedance in the order of 4 mΩ when conducting and an output impedance in the order of several megohms when switched off.

4. The illumination system according to any one of Claims1 to 3, wherein:

the high frequency is an order of several hundred KHz, and

the step down transformer is a planar transformer (50).

5. The illumination system according to any one of Claims 1 to 4, wherein the frequency converter includes a full bridge inverter (15).

6. The illumination system according to any one of the preceding claims, wherein the high frequency, low voltage pulses are square wave pulses.

7. The illumination system according to any one of the preceding claims, wherein the synchronous rectifier is physically separated from the frequency converter and the step down transformer.

8. The illumination system according to any one of the preceding claims, further including a smoothing unit (42) coupled between the low voltage d.c. and the conductors for removing high frequency voltage components from the low voltage d.c.

9. The illumination system according to any one of Claims 1 to 8, further including a protection circuit (55) that includes an impedance measuring unit comprising:

a comparator circuit (60) having fed to a first input thereof (57) a function of a supply voltage associated with the illumination system and having fed to a second input thereof (58) a function of a current flow associated with the illumination system such that an output of the comparator goes from a first state to a second state when a predetermined function of the impedance across the conductors falls below said predetermined threshold.

10. The illumination system according to Claim 9, further including a latching circuit comprising a feedback resistor (60) connected between the output of the comparator circuit and the non-inverting input of the comparator circuit.

11. The illumination system according to any one of Claims 1 to 10, wherein the protection circuit (55) includes an impedance measuring unit associated with the frequency converter for measuring an output impedance of the of a.c. voltage source.

12. The illumination system according to any one of Claims 1 to 11, wherein the protection circuit further includes an indication unit (61) responsively coupled to the impedance measuring unit for providing an indication if said impedance falls below said predetermined threshold.

13. The illumination system according to any one of Claims 1 to 12, wherein the predetermined threshold is a function of lamp voltage.

14. The illumination system according to Claim 9 or 10, wherein the protection circuit further includes a circuit interruption unit responsive to a function of the voltage across the pair of conductors falling below a predetermined threshold for interrupting power to said pair of conductors, whereby power is interrupted towards the start of the a.c. current cycle when the magnitude of the current waveform is still too low to constitute an overcurrent even if the peak value of the waveform is dangerously high.

15. The illumination system according to Claim 9, 10 or 14, including compensation means to compensate for variation in lamp impedance owing to a change in operation voltage.

16. The illumination system according to Claim 15, wherein the compensation means includes a voltage divider comprising a pair of resistors (R, r) having a common junction connected to the inverting input of the comparator circuit (60) and a reference resistor (R_SENSE) coupled to the non-inverting input of the comparator circuit (60), and wherein at least one of said resistors (R, r, R_SENSE) is dependent on the instantaneous value of the lamp voltage.

17. The illumination system according to Claim 16, wherein the at least one of said resistors (R, r, R_SENSE) is a voltage-controlled resistor such as a FET operated in the region before pinch-off, where the drain to source voltage V_DS is small.

18. The illumination system according to Claim 16, wherein the compensation means includes discrete control provided by a bank of equal value resistors connected in parallel which are selectively switched in circuit according to the lamp voltage, each of the resistors being connected in series with a corresponding MOSFET whose gate terminals are driven by a respective threshold comparator having a threshold corresponding to a different preset voltage; whereby the corresponding threshold comparator effects enabling or disabling of the resistors allowing the resistance of the resistor bank to be varied as required.

Ansprüche

1. Niederspannungsbeleuchtungssystem (10) mit:

Verbindungsanschlüssen (11, 12) zum Verbinden mit einer Niederfrequenz-Wechselspannungsquelle (13),

einem Frequenzwandler (16), der mit der Wechselspannungsquelle gekoppelt ist, um die Niederfrequenz-Wechselspannung auf eine Hochfrequenz-Wechselspannung umzuwandeln, die durch eine gleichgerichtete Vollwellen-Niederfrequenz-Hüllkurve moduliert wird,

einem Abspanntransformator (17), der mit einem Ausgang des Frequenzwandlers zum Umwandeln auf eine niedrige Hochfrequenz-Wechselspannung gekoppelt ist,

einem Synchrongleichrichter (18), der mit einer Sekundärwicklung des Abspanntransformators (17) zum Umwandeln der niedrigen Hochfrequenz-Wechselspannung auf eine Sollgleichspannung geringer Größe gekoppelt ist, und

einem Paar Leitungen (19, 20), das mit der Sollgleichspannung geringer Größe verbunden ist, um die Niederspannungslampen damit zu verbinden;

dadurch gekennzeichnet, dass:

der Synchrongleichrichter (18) mindestens zwei MOSFETs (38, 40) aufweist,

der Abspanntransformator (17) mit einem Ausgang des Frequenzwandlers gekoppelt ist, um die MOSFETs so zu steuern, dass gleichgerichtete Hochfrequenz-Niederspannungsimpulse erzeugt werden, die durch eine gleichgerichtete Vollwellen-Niederfrequenz-Hüllkurve moduliert werden, und

eine Rest-Speichereinrichtung (32) in Bezug auf jeden MOSFET bereitgestellt wird, um die Ladung zu speichern, wenn der jeweilige MOSFET AUS ist, so dass deren Gate-Source-Spannung verstärkt wird, wenn der jeweilige MOSFET AN ist, damit der jeweilige MOSFET für einen längeren Zeitraum AN bleibt, wobei dadurch die Sperrzeit an jedem Ende der gleichgerichteten Wechselstrom-Halbwelle zwischen einem MOSFET, der AUS schaltet und einem komplementären MOSFET, der AN schaltet, verringert wird.

2. Beleuchtungssystem nach Anspruch 1, wobei:

der Synchrongleichrichter (18) ein Paar MOSFETs (38, 40) und

der Abspanntransformator (17) eine Mittelanzapfung aufweist, um die MOSFETs zu steuern.

3. Beleuchtungssystem nach Anspruch 1 oder 2, wobei die MOSFETs eine Soll-Ausgangsimpedanz in der Größenordnung von 4 mΩ, wenn sie leiten, und eine Ausgangsimpedanz in der Größenordnung von mehreren Megaohm haben, wenn sie ausgeschaltet sind.

4. Beleuchtungssystem nach einem der Ansprüche 1 bis 3, wobei:

die Hochfrequenz eine Größenordnung von mehreren Hundert KHz hat, und

der Abspanntransformator ein Flächentransformator (50) ist.

5. Beleuchtungssystem nach einem der Ansprüche 1 bis 4, wobei der Frequenzwandler einen Vollbrücken-Wechselrichter (15) aufweist.

6. Beleuchtungssystem nach einem der vorhergehenden Ansprüche, wobei die Hochfrequenz-Niederspannungsimpulse Rechteckwellenimpulse sind.

7. Beleuchtungssystem nach einem der vorhergehenden Ansprüche, wobei der Synchrongleichrichter von dem Frequenzwandler und dem Abspanntransformator physikalisch getrennt ist.

8. Beleuchtungssystem nach einem der vorhergehenden Ansprüche, das des Weiteren eine Glättungseinheit (42) aufweist, die zwischen der niedrigen Gleichspannung und den Leitungen gekoppelt ist, um die Hochfrequenz-Spannungskomponenten von der niedrigen Gleichspannung zu entfernen.

9. Beleuchtungssystem nach einem der Ansprüche 1 bis 8, das ferner eine Schutzschaltung (55) aufweist, die eine Impedanz-Messeinheit aufweist, mit:

einer Komparatorschaltung (60), an deren ersten Eingang (57) eine Funktion einer Versorgungsspannung eingespeist wird, die mit dem Beleuchtungssystem verbunden ist, und an deren zweiten Eingang (58) eine Funktion eines Stromflusses eingespeist wird, die mit dem Beleuchtungssystem verbunden ist, so dass ein Ausgang des Komparators von einem ersten Zustand auf einen zweiten Zustand übergeht, wenn eine vorgegebene Funktion der Impedanz über den Leitungen unter den vorgegebenen Schwellenwert fällt.

10. Beleuchtungssystem nach Anspruch 9, das ferner einen Haltestromkreis mit einem Rückkopplungswiderstand (60) aufweist, der zwischen dem Ausgang der Komparatorschaltung und dem nicht invertierenden Eingang der Komparatorschaltung geschaltet ist.

11. Beleuchtungssystem nach einem der Ansprüche 1 bis 10, wobei die Schutzschaltung (55) eine Impedanz-Messeinheit aufweist, die mit dem Frequenzwandler verbunden ist, um eine Ausgangsimpedanz der Wechselspannungsquelle zu messen.

12. Beleuchtungssystem nach einem der Ansprüche 1 bis 11, wobei die Schutzschaltung ferner eine Anzeigeeinheit (61) aufweist, die mit der Impedanz-Messeinheit reagierend gekoppelt ist, um eine Anzeige bereitzustellen, wenn die Impedanz unter den vorgegebenen Schwellenwert fällt.

13. Beleuchtungssystem nach einem der Ansprüche 1 bis 12, wobei der vorgegebene Schwellenwert eine Funktion der Lampenspannung ist.

14. Beleuchtungssystem nach Anspruch 9 oder 10, wobei die Schutzschaltung ferner eine Schaltungsunterbrechungseinheit aufweist, die auf eine Funktion der Spannung über dem Leitungspaar reagiert, die unter einen vorgegebenen Schwellenwert fällt, um die Energie zu dem Leitungspaar zu unterbrechen, wodurch die Energie zum Beginn des Wechselstromzyklus unterbrochenen wird, wenn die Größe der Strom-Wellenform noch zu niedrig ist, um einen Überstrom zu bilden, selbst wenn der Spitzenwert der Wellenform gefährlich hoch ist.

15. Beleuchtungssystem nach Anspruch 9, 10 oder 14, das eine Ausgleichseinrichtung aufweist, um eine Schwankung der Lampenimpedanz infolge einer Änderung der Betriebsspannung auszugleichen.

16. Beleuchtungssystem nach Anspruch 15, wobei die Ausgleichseinrichtung einen Spannungsteiler aufweist, der ein Paar Widerstände (R, r) mit einem gemeinsamen Verbindungspunkt, der mit dem invertierenden Eingang der Komparatorschaltung (60) verbunden ist, und einen Bezugswiderstand (R_SENSE) aufweist, der mit dem nicht invertierenden Eingang der Komparatorschaltung (60) gekoppelt ist, und wobei mindestens einer der Widerstände (R, r, R_SENSE) vom augenblicklichen Wert der Lampenspannung abhängt.

17. Beleuchtungssystem nach Anspruch 16, wobei mindestens einer der Widerstände (R, r, R_SENSE) ein spannungsgesteuerter Widerstand wie ein FET ist, der im Bereich vor der Abschnürung betrieben wird, in dem die Drain-Source-Spannung V_DS klein ist.

18. Beleuchtungssystem nach Anspruch 16, wobei die Ausgleichseinrichtung eine diskrete Steuerung aufweist, die durch eine Bank parallel geschalteter Widerstände mit gleichem Wert bereitgestellt wird, die in der Schaltung entsprechend der Lampenspannung selektiv geschaltet werden, wobei jeder der Widerstände in Reihe mit einem entsprechenden MOSFET geschaltet ist, dessen Gate-Anschlüsse durch einen jeweiligen Schwellenwert-Komparator gesteuert werden, mit einem Schwellenwert, der einer unterschiedlichen, vorher eingestellten Spannung entspricht; wodurch der entsprechende Schwellenwert-Komparator die Freigabe oder das Sperren der Widerstände bewirkt, wobei es ermöglicht wird, den Widerstand der Widerstandsbank nach Bedarf zu verändern.

Revendications

1. Système d'éclairage basse tension (10), comprenant :

des bornes de connexion (11, 12) pour la connexion à une source (13) de tension alternative à basse fréquence,

un convertisseur de fréquence (16) couplé à la source de tension alternative pour convertir la tension alternative à basse fréquence en tension alternative à haute fréquence, modulée par une enveloppe basse fréquence redressée pleine onde,

un transformateur abaisseur (17) couplé à une sortie du convertisseur de fréquence pour convertir en courant alternatif basse tension à haute fréquence,

un redresseur synchrone (18) couplé à un secondaire du transformateur abaisseur (17) pour convertir le courant alternatif basse tension à haute fréquence en une tension continue de basse grandeur nominale, et

une paire de conducteurs (19, 20) connectés à la tension continue de basse grandeur nominale pour connecter des lampes basse tension à celle-ci ;

   caractérisé en ce que :

le redresseur synchrone (18) comprend au moins deux transistors à effet de champ MOS (38, 40),

le transformateur abaisseur (17) est couplé à une sortie du convertisseur de fréquence pour commander lesdits transistors à effet de champ MOS de manière à produire des impulsions basse tension à haute fréquence redressées modulées par une enveloppe basse fréquence redressée pleine onde, et

un moyen de stockage résiduel (32) est disposé par rapport à chaque transistor à effet de champ MOS pour emmagasiner une charge quand le transistor à effet de champ MOS respectif est à l'état bloqué de manière à élever une tension grille-source de celui-ci quand le transistor à effet de champ MOS respectif est à l'état passant, afin que le transistor à effet de champ MOS respectif reste à l'état passant pendant une période de temps étendue en réduisant ainsi le temps de coupure à chaque fin de la demi-période de courant alternatif redressé entre une commutation à l'état bloqué d'un transistor à effet de champ MOS et une commutation à l'état passant d'un transistor à effet de champ MOS complémentaire.

2. Système d'éclairage selon la revendication 1, dans lequel :

le redresseur synchrone (18) comprend une paire de transistors à effet de champ MOS (3 8, 40), et

le transformateur abaisseur (17) comprend une prise centrale pour commander les transistors à effet de champ MOS.

3. Système d'éclairage selon la revendication 1 ou 2, dans lequel les transistors à effet dé champ MOS ont une impédance de sortie nominale de l'ordre de 4 mΩ quand ils sont passants et une impédance de sortie de l'ordre de plusieurs mégohms quand ils sont commutés à l'état bloqué.

4. Système d'éclairage selon l'une quelconque des revendications 1 à 3, dans lequel
   la haute fréquence est de l'ordre de plusieurs centaines de KHz, et
   le transformateur abaisseur est un transformateur plan (50).

5. Système d'éclairage selon l'une quelconque des revendications 1 à 4, dans lequel le convertisseur de fréquence comprend un onduleur à pont intégral (15).

6. Système d'éclairage selon l'une quelconque des revendications précédentes, dans lequel les impulsions basse tension à haute fréquence sont des impulsions d'ondes rectangulaires.

7. Système d'éclairage selon l'une quelconque des revendications précédentes, dans lequel le redresseur synchrone est physiquement séparé du convertisseur de fréquence et du transformateur abaisseur.

8. Système d'éclairage selon l'une quelconque des revendications précédentes, comprenant en outre une unité de lissage (42) couplée entre le courant continu basse tension et les conducteurs pour éliminer des composantes de tension haute fréquence du courant continu basse tension.

9. Système d'éclairage selon l'une quelconque des revendications 1 à 8, comprenant en outre un circuit de protection (55) qui comprend une unité de mesure d'impédance comprenant :

un circuit comparateur (60) recevant sur une première entrée de celui-ci (57) une fonction d'une tension d'alimentation associée au système d'éclairage et recevant sur une deuxième entrée de celui-ci (58) une fonction d'un flux de courant associé au système d'éclairage de manière qu'une sortie du comparateur passe d'un premier état à un deuxième état quand une fonction prédéterminée de l'impédance aux bornes des conducteurs chute au-dessous d'un seuil prédéterminé.

10. Système d'éclairage selon la revendication 9, comprenant en outré un circuit de verrouillage comprenant une résistance de rétroaction (60) connectée entre la sortie du circuit comparateur et l'entrée de non inversion du circuit comparateur.

11. Système d'éclairage selon l'une quelconque des revendications 1 à 10, dans lequel le circuit de protection (55) comprend une unité de mesure d'impédance associée au convertisseur de fréquence pour mesurer une impédance de sortie de la source de tension alternative.

12. Système d'éclairage selon l'une quelconque des revendications 1 à 11, dans lequel le circuit de protection comprend en outre une unité d'indication (61) couplée de manière sensible à l'unité de mesure d'impédance pour fournir une indication si ladite impédance descend au-dessous dudit seuil prédéterminé.

13. Système d'éclairage selon l'une quelconque des revendications 1 à 12, dans lequel le seuil prédéterminé est une fonction de tension de lampe.

14. Système d'éclairage selon la revendication 9 ou 10, dans lequel le circuit de protection comprend en outre une unité de coupure de circuit sensible à une fonction de la tension aux bornes de la paire de conducteurs chutant au-dessous d'un seuil prédéterminé pour interrompre l'énergie sur ladite paire de conducteurs, moyennant quoi l'énergie est interrompue vers le début d'une période de courant alternatif quand la grandeur de la forme d'onde de courant est encore trop basse pour constituer une surintensité même si la valeur de crête de la forme d'onde est dangereusement élevée.

15. Système d'éclairage selon la revendication 9, 10 ou 14, comprenant des moyens de compensation pour compenser une variation de l'impédance de lampe due à un changement de tension de fonctionnement.

16. Système d'éclairage selon la revendication 15, dans lequel les moyens de compensation comprennent un diviseur de tension comprenant une paire de résistances (R, r) ayant une jonction commune connectée à l'entrée d'inversion du circuit comparateur (60) et une résistance de référence (R_SENSE) couplée à l'entrée de non inversion du circuit comparateur (60) et dans lequel au moins une desdites résistances (R, r, R_SENSE) est dépendante de la valeur instantanée de la tension de lampe.

17. Système d'éclairage selon la revendication 16, dans lequel ladite au moins une desdites résistances (R, r, R_SENSE) est une résistance commandée par tension telle qu'un transistor à effet de champ qui est mise en oeuvre dans la région avant pincement, où la tension drain-source V_DS est faible.

18. Système d'éclairage selon la revendication 16, dans lequel les moyens de compensation comprennent une commande discrète fournie par une batterie de résistances de valeur égale connectées en parallèle, qui sont commutées en circuit de manière sélective selon la tension de lampe, chacune des résistances étant connectée en série avec un transistor à effet de champ MOS correspondant dont les bornes de grille sont commandées par un comparateur de seuil respectif ayant un seuil correspondant à une tension prédéterminée différente ; moyennant quoi le comparateur de seuil correspondant effectue la validation ou l'invalidation des résistances en permettant de faire varier la résistance de la batterie de résistances comme nécessaire.

Drawing